Semiconductor photodetector and avalanche photodiode

ABSTRACT

There is provided a semiconductor photodetector which comprises (i)an InP substrate( 1 ), (ii)an optical waveguide( 5 ) having an N-type semiconductor layer( 32 ) formed on the InP substrate( 1 ), an optical waveguide core layer( 3 ) formed on a partial area of the N-type semiconductor layer( 32 ), and an upper cladding layer( 4 ) formed on the optical waveguide core layer( 3 ), and (iii)an avalanche photodiode( 17 ) constructed by forming a photo absorbing layer( 33 ), a heterobarrier relaxing layer( 34 ), an underlying layer( 14   a ) of a N-type field dropping layer( 35 ), an overlying layer( 14   b ) of the N-type field dropping layer( 35 ), a carrier multiplying layer( 36 ), and a P-type semiconductor layer( 37 ) in sequence on another area of the N-type semiconductor layer( 32 ), and coupled to the optical waveguide( 5 ), wherein a side surface of the underlying layer( 14   a ) of the N-type field dropping layer( 35 ) comes into contact with a side surface of the optical waveguide core layer( 3 ), and a part of the overlying layer( 14   b ) of the N-type field dropping layer( 35 ) is formed on the optical waveguide core layer( 3 ).

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is based upon and claims priority of JapanesePatent Applications No. 2002-316506, filed on Oct. 30, 2002, and No.2003-167793, filed on Jun. 12, 2003, the contents being incorporatedherein by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a semiconductor photodetectorand an avalanche photodiode, which are employed in a high-speed opticalcommunication, etc.

[0004] 2. Description of the Prior Art

[0005] A sectional structure of a semiconductor photodetector for thevery high-speed optical communication in the prior art is shown in FIG.1.

[0006] In the structure in FIG. 1, an N-type semiconductor layer 2 madeof N-type InP, an optical waveguide core layer 3 made of undopedInGaAsP, and an upper cladding layer 4 made of undoped InP are formed inthis sequence on a predetermined region of an undoped InP substrate 1,and these layers constitute an optical waveguide 5. Then, a photoabsorbing layer 6 made of undoped InGaAs and a P-type semiconductorlayer 7 made of P-type InP are formed in this sequence on the N-typesemiconductor layer 2 located near an end portion of the opticalwaveguide 5, these layers as well as the N-type semiconductor layer 2constitute a PIN photodiode 8.

[0007] In this semiconductor photodetector, in order to attain asufficient quantum efficiency in the photo absorbing layer 6 whosethickness is thin, the light emitted from the optical waveguide corelayer 3 is incident upon the photo absorbing layer 6 from the paralleldirection to the jointed surface between the N-type semiconductor layer2 and the photo absorbing layer 6. Assume that such structure isreferred to as a “lateral-type incident structure” hereinafter. Suchstructure, although not disclosed, is set forth in Japanese PatentApplication No.2002-214408.

[0008] Then, when the light is input in the state that a reverse biasvoltage is applied to an N-side electrode 9 and a P-side electrode 10,pairs of electron-hole are produced in the photo absorbing layer 6 andthus a photo current is sensed.

[0009] In this case, as the photodiode, in addition to the above PINphotodiode, the avalanche photodiode (APD) set forth in PatentLiterature 1 is also known. In addition, not the lateral-type incidentstructure but the planar-type avalanche photodiode having the guard ringstructure is disclosed in Patent Literature 2, for example. Then, theoperating characteristic of the avalanche photodiode is disclosed inNon-Patent Literature 1.

Patent Literature 1

[0010] Patent Application Publication (KOKAI) Hei 11-354827

Patent Literature 2

[0011] Patent Application Publication (KOKAI) Hei 10-209486

Non-Patent Literature 1

[0012] R. B. Emmons, J. Appl. Phys. 38, 3705, 1967

[0013] In the semiconductor photodetector having the lateral-typeincident structure, it is desired that the further large photo currentshould be extracted and also the receiving sensitivity should beenhanced. For this reason, it is preferable that the photodiode that isdifferent from the PIN photodiode 8 should be employed.

[0014] Further, in such avalanche photodiode, it is preferable that notonly simply the receiving sensitivity can be increased but also anoperation at a high speed should be performed in a manner such that thelight signal being modulated at a high speed can be received and thenthe photo current that follows up such light signal can be obtained.

SUMMARY OF THE INVENTION

[0015] The present invention has been made in light of the problems inthe prior art, and it is an object of the present invention to provide asemiconductor photodetector capable of extracting a photo current thatis larger than that in the prior art and having a high receivingsensitivity.

[0016] Also, it is another object of the present invention to provide anavalanche photodiode capable of operating at a speed that is higher thanthat in the prior art.

[0017] According to an aspect of the present invention, there isprovided a semiconductor photodetector which comprises a semiconductorsubstrate; an optical waveguide having a first conductivity typesemiconductor layer formed on the semiconductor substrate, an opticalwaveguide core layer formed on a partial area of the first conductivitytype semiconductor layer, and an upper cladding layer formed on theoptical waveguide core layer; and an avalanche photodiode constructed byforming a photo absorbing layer, a heterobarrier relaxing layer, anunderlying layer of a first conductivity type field dropping layer, anoverlying layer of the first conductivity type field dropping layer, acarrier multiplying layer, and a second conductivity type semiconductorlayer in sequence on another area of the first conductivity typesemiconductor layer, and coupled to the optical waveguide; wherein aside surface of the underlying layer of the first conductivity typefield dropping layer comes into contact with a side surface of theoptical waveguide core layer, and a part of the overlying layer of thefirst conductivity type field dropping layer is formed on the opticalwaveguide core layer.

[0018] Next, advantages of the present invention will be explainedhereunder.

[0019] According to the present invention, the semiconductorphotodetector having the lateral-type incident structure is providedwith the avalanche photodiode (APD) having the larger multiplicationfactor than the PIN photodiode. Therefore, the photo current that islarger than the case where the PIN photodiode is employed can beextracted and also the receiving sensitivity can be enhanced.

[0020] In addition, according to the present invention, the side surfaceof the underlying layer of the first conductivity type field droppinglayer comes into contact with the side surface of the optical waveguidecore layer, and a part of the overlying layer of the first conductivitytype field dropping layer is formed on the optical waveguide core layer.Therefore, as for the electric field distribution in the depth directionof APD, the electric field is dropped by two layers of the overlyinglayer and the underlying layer of the first conductivity type fielddropping layer near the center portion of APD whereas the dropping ofthe electric field is caused only by one layer of the overlying layer ofthe first conductivity type field dropping layer in the cross sectionincluding the optical waveguide core layer. As a result, with respect tothe value obtained by integrating the electric field with respect to thedepth of APD, the value in the cross section including the opticalwaveguide core layer becomes larger than the value in the cross sectionaround the center portion of APD. Normally, the breakdown voltage of APDis increased higher as the above integral value becomes larger.Therefore, according to the above, the breakdown voltage becomes high atthe edge portion of APD in contrast to the center portion and thus thebreakdown at the edge portion of APD is hard to occur.

[0021] In addition, composition of the constituent element in theunderlying layer of the first conductivity type field dropping layer ischanged from its lower surface to its upper surface to cause theunderlying layer to function as the heterobarrier relaxing layer. Thisresults in preventing the event that the carrier is trapped by theunderlying layer to lower the response speed of the semiconductorphotodetector.

[0022] Also, according to another aspect of the present invention, thereis provided an avalanche photodiode which comprises a semiconductorsubstrate; a photo absorbing layer formed over the semiconductorsubstrate and a film thickness of which is set to more than 0.15 μm butless than 0.2 μm; and a carrier multiplying layer formed over the photoabsorbing layer and a film thickness of which is set to more than 0.07μm but less than 0.1 μm.

[0023] Next, advantages of the present invention will be explainedhereunder.

[0024] According to the results of the experiment carried out by theinventors of this application, it was validated that, like the presentinvention, if the film thickness of the photo absorbing layer is set tomore than 0.15 μm but less than 0.2 μm and the film thickness of thecarrier multiplying layer is set to more than 0.07 μm but less than 0.1μm, the value of the MB product of the avalanche photodiode can beincreased more largely than the value that is predicted theoretically.If the value of the MB product is increased in this manner, theoperating speed of the avalanche photodiode can be accelerated, whichcan contribute largely to the higher speed of the optical communication.

BRIEF DESCRIPTION OF THE DRAWINGS

[0025]FIG. 1 is a sectional view showing a semiconductor photodetectorin the prior art;

[0026]FIG. 2 is a sectional view showing a structure in which a PINphotodiode of the semiconductor photodetector in the prior art is simplyreplaced with an avalanche photodiode;

[0027]FIG. 3 is a sectional view showing a semiconductor photodetectoraccording to a first embodiment of the present invention;

[0028]FIG. 4 is a sectional view (#1) showing steps of manufacturing thesemiconductor photodetector according to the first embodiment of thepresent invention;

[0029]FIG. 5 is a sectional view (#2) showing steps of manufacturing thesemiconductor photodetector according to the first embodiment of thepresent invention;

[0030]FIG. 6 is a perspective view showing steps of manufacturing thesemiconductor photodetector according to the first embodiment of thepresent invention;

[0031]FIG. 7 is a sectional view (#3) showing steps of manufacturing thesemiconductor photodetector according to the first embodiment of thepresent invention;

[0032]FIG. 8 is a sectional view (#4) showing steps of manufacturing thesemiconductor photodetector according to the first embodiment of thepresent invention;

[0033]FIG. 9 is a sectional view (#5) showing steps of manufacturing thesemiconductor photodetector according to the first embodiment of thepresent invention;

[0034]FIG. 10 is a sectional view (#6) showing steps of manufacturingthe semiconductor photodetector according to the first embodiment of thepresent invention;

[0035]FIG. 11 is a plan view (#1) showing steps of manufacturing thesemiconductor photodetector according to the first embodiment of thepresent invention;

[0036]FIG. 12 is a sectional view (#7) showing steps of manufacturingthe semiconductor photodetector according to the first embodiment of thepresent invention;

[0037]FIG. 13 is a plan view (#2) showing steps of manufacturing thesemiconductor photodetector according to the first embodiment of thepresent invention;

[0038]FIG. 14 is a perspective view showing the semiconductorphotodetector according to the first embodiment of the presentinvention;

[0039]FIG. 15 is a graph showing an electric field distribution in thedepth direction of the semiconductor photodetector according to thefirst embodiment of the present invention;

[0040]FIG. 16 is a view showing the electric field distribution in bothcases where a carrier multiplying layer in the semiconductorphotodetector according to the first embodiment of the present inventionis thinned and is not thinned;

[0041]FIG. 17 is a sectional view showing an avalanche photodiodeaccording to a second embodiment of the present invention;

[0042]FIG. 18 is a graph showing a relationship between a film thicknessof the carrier multiplying layer and an MB product, in the avalanchephotodiode according to the second embodiment of the present invention;

[0043]FIG. 19 is a graph showing a relationship between the filmthickness of the carrier multiplying layer and a tunnel current, in theavalanche photodiode according to the second embodiment of the presentinvention;

[0044]FIG. 20 is a graph showing a film thickness of a photo absorbinglayer and the MB product, in the avalanche photodiode according to thesecond embodiment of the present invention; and

[0045]FIG. 21 is a graph showing a relationship between the filmthickness of the photo absorbing layer and a quantum efficiency, in theavalanche photodiode according to the second embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0046] Embodiments of the present invention will be explained withreference to the drawings hereinafter.

FIRST EMBODIMENT

[0047] Prior to the explanation of present embodiments, preliminarymatters as the basis of the present invention will be explainedhereunder.

[0048] In the semiconductor photodetector having the lateral-typeincident structure, in order to enhance the receiving sensitivity andextract the large photo current, it may be considered to employ theavalanche photodiode that can have a higher current-amplifying actionthan the PIN photodiode and can receive the light signal, which ismodulated at a high speed, at a high quantum efficiency.

[0049] If the PIN photodiode 8 shown in FIG. 1 is replaced simply withthe avalanche photodiode, the lateral-type incident structure shown inFIG. 2 is derived. In this case, in FIG. 2, the same references as thosein FIG. 1 are affixed to the same members as those in FIG. 1.

[0050] An avalanche photodiode 17 in FIG. 2 is constructed by forming aphoto absorbing layer 12 made of undoped InGaAs, a heterobarrierrelaxing layer 13 made of undoped InGaAsP, a field dropping layer (firstconductivity type field dropping layer) 14 made of N-type InP, a carriermultiplying layer 15 made of undoped InP, and a P-type InP layer (secondconductivity type semiconductor layer) 16 in this order on the N-typesemiconductor layer (first conductivity type semiconductor layer) 2.

[0051] In this case, a band gap of the heterobarrier relaxing layer 13is larger than a band gap of the photo absorbing layer 12, and a bandgap of the carrier multiplying layer 15 is larger than a band gap of theheterobarrier relaxing layer 13.

[0052] In this case, in the present embodiment, as the case may be,silicon may be doped into the InP substrate 1.

[0053] Also, an N-side electrode 18 made of Ti/Pt/Au is formed on theN-type semiconductor layer 2, and a P-side electrode 19 made of Ti/Pt/Auis formed on the P-type InP layer 16. A reverse bias voltage is appliedbetween these electrodes.

[0054] Out of above layers, the heterobarrier relaxing layer 13 isformed such that compositions of constituent elements are changedgradually from InGaAs to InP, and functions to relax the heterobarrierbetween the photo absorbing layer 12 and the field dropping layer 14 andalso prevent the holes from being trapped by the heterobarrier.

[0055] Also, the field dropping layer 14 functions to cause abruptly adrop of the electric field therein and apply the high electric field tothe overlying carrier multiplying layer 15.

[0056] The light that propagates through the optical waveguide corelayer 3 is incident upon the photo absorbing layer 12, and then produceselectron-hole pairs therein. The holes out of these pairs flow into thecarrier multiplying layer 15. Since the high electric field is appliedto the carrier multiplying layer 15 as described above, the injectedholes cause the ionization successively, so that the carriermultiplication is carried out. As a result, the photo current that islarger than the case where the PIN photodiode is employed can beobtained.

[0057] However, in this structure, since the electric field isconcentrated into the field dropping layer 14 near the boundary to theoptical waveguide 5 (A portion in FIG. 2), the breakdown is ready tooccur at that portion. Then, it causes a disadvantage such that the darkcurrent is increased, or a disadvantage such that multiplication of theavalanche photodiode 17 cannot be set largely because the high voltagecannot be applied between the N-side electrode 18 and the P-sideelectrode 19.

[0058] In view of these respects, the inventors of the present inventionthought of a semiconductor photodetector shown in FIG. 3. In FIG. 3, thesame references as those in FIG. 2 are affixed to the same members asthose in FIG. 2, and their explanation will be omitted herein.

[0059] Different respects of this photodetector from the device in FIG.2 reside in that the field dropping layer 14 consists of an underlyinglayer 14 a and an overlying layer 14 b and that the optical waveguidecore layer 3 comes into an area that is located below the overlyinglayer 14 b. Since the underlying layer 14 a has the composition that ischanged gradually from composition of the heterobarrier relaxing layer13 to composition of the overlying layer 14 b upwardly from its lowersurface to its upper surface, such underlying layer 14 a also functionsas the heterobarrier relaxing layer. That results in preventingreduction in a response speed caused because the holes are trapped bythe underlying layer 14 a. Such underlying layer 14 a is formed ofN-type InGaAsP, for example, while such overlying layer 14 b is formedof InP, for example.

[0060]FIG. 15 is a graph showing an electric field distribution in thedepth direction of such semiconductor photodetector, wherein an ordinatedenotes a depth from the avalanche photodiode 17 and an abscissa denotesan electric field strength E at that depth. Then, a solid line shows theelectric field distribution along a I-I line in FIG. 3, and a dot-dashline shows the electric field distribution along a II-II line in FIG. 3.

[0061] As shown in FIG. 15, in the case of the distribution taken alonga II-II line, the electric field is dropped by two layers of theunderlying layer 14 a and the overlying layer 14 b. In contrast, in thecase of the distribution taken along a I-I line, because the underlyinglayer 14 a is not provided, the electric field is dropped merely by onelayer of the overlying layer 14 b. As a result, with regard to an areain the graph which is derived by integrating the electric field E withrespect to the depth, the area in the case taken along a I-I linebecomes larger than that in the case taken along a II-II line.

[0062] Normally, it is known that the breakdown voltage of the avalanchephotodiode becomes higher as the above area becomes larger. Therefore,in this case, the breakdown voltage in the case taken along a I-I linebecomes higher than the breakdown voltage in the case taken along aII-II line.

[0063] Accordingly, the breakdown is hard to occur near the boundary tothe optical waveguide 5. Therefore, the sufficiently high voltage can beapplied between the electrodes 18, 19, and not only multiplication inthe avalanche photodiode 17 can be achieved more largely than that inFIG. 2 but also the dark current can be reduced.

[0064] Next, steps of manufacturing the semiconductor photodetector, asdescribed above, will be explained with reference to FIG. 4 to FIG. 13hereunder.

[0065] At first, steps required until a sectional structure shown inFIG. 4A is obtained will be explained hereunder.

[0066] First, N-type InP, into which Fe as the N-type impurity is dopedat 1×10¹⁸ cm⁻³, is epitaxially grown on an InP substrate (semiconductorsubstrate) 1, into which silicon is doped, to have a thickness of about2 μm. This layer is used as an N-type semiconductor layer 2. Forexample, the MOCVD method is employed in this epitaxial growth. Also,layers manufactured by following steps are formed by the MOCVD method.

[0067] Then, an undoped InGaAs layer 12 a of about 0.5 μm thickness isepitaxially grown on the N-type semiconductor layer 2. In order toachieve the lattice matching between the undoped InGaAs layer 12 a andthe underlying N-type semiconductor layer 2 made of InP, a compositionratio of In and Ga is set to In:Ga=0.53:0.47 in the undoped InGaAs layer12 a.

[0068] Then, an undoped InGaAs_(x)P_(1-x) layer 13 a is epitaxiallygrown on the undoped InGaAs layer 12 a. A composition wavelength of theundoped InGaAs_(x)P_(1-x) layer 13 a is 1.25 μm, and a thickness thereofis about 0.18 μm, and an x value thereof becomes small gradually from 1.

[0069] In addition, an N-type InGaAs_(y)P_(1-y) layer 14 c isepitaxially grown on the InGaAs_(x)P_(1-x) layer 13 a to have athickness of about 0.02 μm. In the N-type InGaAs_(y)P_(1-y) layer 14 c,Si is doped up to 1×10¹⁸ cm⁻³ as the N-type impurity and the y value isreduced gradually and becomes 0 finally. A composition wavelength ofthis InGaAs_(y)P_(1-y) layer 14 c is 1.25 μm. In this case, in order toachieve the lattice matching between the InGaAs_(x)P_(1-x) layer 13 aand the N-type InGaAs_(y)P_(1-y) layer 14 c, x=y is set at the boundarybetween these layers.

[0070] Then, an N-type InP layer 14 d, into which Si is doped up to1×10¹⁸ cm⁻³ as the N-type impurity, is epitaxially grown on the N-typeInGaAs_(y)P_(1-y) layer 14 c to have a thickness of about 0.02 μm.

[0071] Then, an InP layer 15 a of about 0.20 μm thickness is epitaxiallygrown on this N-type InP layer 14 d. Then, a P-type InP layer 16 a, intowhich Zn is doped by 1×10¹⁸ cm⁻³ as the P-type impurity, is epitaxiallygrown thereon.

[0072] Then, photoresist is formed on this P-type InP layer 16 a. Then,a resist pattern 20 having an almost rectangular planar shape is formedby exposing/developing this photoresist.

[0073] Then, while using this resist pattern 20 as an etching mask andusing a mixed solution consisting of sulfuric acid and hydrogen peroxidesolution as an etchant, respective layers from the P-type InP layer 16 ato the undoped InGaAs layer 12 a are etched.

[0074] Accordingly, as shown in FIG. 4B, the undoped InGaAs layer 12 aserves as the photo absorbing layer 12, and the undopedInGaAs_(x)P_(1-x) layer 13 a serves as the heterobarrier relaxing layer13. Also, the N-type InGaAs_(y)P_(1-y) layer 14 c serves as theunderlying layer 14 a of the field dropping layer 14, and the N-type InPlayer 14 d serves as the overlying layer 14 b of the field droppinglayer 14. Then, the InP layer 15 a and the P-type InP layer 16 a serveas the carrier multiplying layer 15 and the P-type semiconductor layer16 respectively.

[0075] In such etching, etching rates of the underlying layer 14 a, theheterobarrier relaxing layer 13, and the photo absorbing layer 12 arehigher than the etching rate of the overlying layer 14 b. Therefore,respective side surfaces of the underlying layer 14 a, the heterobarrierrelaxing layer 13, and the photo absorbing layer 12 recede inward ratherthan side surfaces of the overlying layer 14 b. The resist pattern 20 isremoved after this etching is completed.

[0076] Next, steps required until a sectional structure shown in FIG. 5is obtained will be explained hereunder.

[0077] First, an undoped InGaAsP layer whose composition wavelength is1.1 μm is epitaxially grown on the N-type semiconductor layer 2 to havea thickness of about 0.7 μm. This layer is used as the optical waveguidecore layer 3. This optical waveguide core layer 3 is constructed toextend under the overlying layer 14 b of the field dropping layer 14 bya distance D (=about 0.2 μm).

[0078] Then, an undoped InP layer 21 of about 2.0 μm thickness isepitaxially grown on the optical waveguide core layer 3.

[0079] Then, as shown in a perspective view of FIG. 6, respective layersformed on the InP substrate 1 as described above are patterned like astripe along the light traveling direction.

[0080] Then, as shown in FIG. 7A, an SiO₂ film 22 for covering edgeportions of the undoped InP layer 21 and the P-type semiconductor layer16 is formed by the thermal CVD method to have a thickness of about 0.1μm.

[0081] Then, as shown in FIG. 7B, portions of the undoped InP layer 21,which are not covered with the SiO₂ film 22, are etched/removedselectively by the wet etching using an HCl solution as an etchant whileusing the SiO₂ film 22 as an etching mask.

[0082] Then, as shown in FIG. 8A, an undoped InP layer of about 2 μmthickness is epitaxially grown on side surfaces of the remaining undopedInP layer 21 and on the optical waveguide core layer 3. This layer isused as the upper cladding layer 4.

[0083] Then, as shown in FIG. 8B, a surface of the P-type semiconductorlayer 16 is exposed by removing the SiO₂ film 22. Then, a Ti film ofabout 3 nm thickness, a Pt film of about 200 nm thickness, and an Aufilm of about 2 μm thickness are formed thereon in this order by thevapor deposition method. These films constitute the P-side electrode 19.A jointed area between the P-side electrode 19 and the P-typesemiconductor layer 16 is set to 6 μm×9 μm.

[0084] Then, as shown in FIG. 9A, respective layers from the uppercladding layer 4 to a part of the InP substrate 1, which are formed onthe opposite side to the light incident side, are removed by executingthe dry etching in the SiF₄ atmosphere. Thus, a surface of the N-typesemiconductor layer 2 is exposed and also a stepped surface 1 a isformed on the InP substrate 1.

[0085] Then, as shown in FIG. 9B, an AuGe film and an Au film arelaminated in this order on the stepped surface 1 a and surface of theN-type semiconductor layer 2 by the vapor deposition method. These filmsconstitute the N-side electrode 18.

[0086] Then, as shown in FIG. 10, an SiN film 23 of about 0.3 μmthickness is formed on an overall surface by the CVD method.

[0087] In this case, a plan view obtained by the steps executed up tonow is shown in FIG. 11. In FIG. 11, the SiN film 23, the upper claddinglayer 4, and the InP layer 21 are omitted. Here, above FIG. 10corresponds to a sectional view taken along a III-III line in FIG. 11.

[0088] As shown in FIG. 11, two N-side electrodes 18 are formed and havea hook-like planar shape respectively.

[0089] Then, as shown in FIG. 12, a hole 23 a is formed in the SiN film23 on the P-side electrode 19. Then, a Ti/Au film is formed in the hole23 a and on a portion of the SiN film 23, which extends from the hole 23a to an area over the stepped surface la. This film is used as a P-sideelectrode leading wiring 24.

[0090] A plan view of the structure obtained by the steps executed up tonow is shown in FIG. 13. In FIG. 13, the SiN film 23, the upper claddinglayer 4, and the InP layer 21 are omitted. Here, above FIG. 12corresponds to a sectional view taken along a IV-IV line in FIG. 13.

[0091] With the above, the semiconductor photodetector according to thepresent embodiment is completed. A perspective view of thissemiconductor photodetector is given as shown in FIG. 14. When a voltageof almost 30 V is applied between the P-side electrode leading wiring 24and the N-side electrode, a sufficient multiplication factor that is inexcess of 10 times can be obtained.

SECOND EMBODIMENT

[0092] In the above-mentioned first embodiment, the semiconductorphotodetector incorporating the avalanche photodiode 17 into thelateral-type incident structure can have the receiving sensitivity thatis higher than that obtained by using the PIN photodiode.

[0093] Meanwhile, in the semiconductor photodetector used in the trunknetwork of the optical communication, it is preferable that not only ithas the high receiving sensitivity in this manner, but also it should beoperated at a high speed so that it can receive the light signalmodulated at a high transmission rate such as about 40 Gbit/sec and thenit can generate the photo current which follows up such light signal.

[0094] In the present embodiment, an explanation will be performed withrespect to an avalanche photodiode that is capable of receiving thelight signal being modulated at a high speed in this manner.

[0095] First of all, an explanation will be performed hereunder withrespect to basic matters about the high speed characteristic of theavalanche photodiode.

[0096] The high speed characteristic of the avalanche photodiode isdisclosed in Non-Patent Literature 1, for example. A 3-dB cut-offfrequency f_(3dB) of the avalanche photodiode in a multiplication risetime is represented therein by a following Equation.

f _(3dB)=1/(2πM)×N(k)k×v/w   (1)

[0097] In Equation (1), M is a multiplication factor, k is a ratio ofionization rates of the electron and the hole in the carrier multiplyinglayer, v is a saturation velocity of the carrier in the carriermultiplying layer, w is a film thickness of the carrier multiplyinglayer, and N(k) is a dimensionless function that changes slowly withrespect to k and is normally approximated to a constant value.

[0098] As shown in Equation (1), since the 3-dB cut-off frequencyf_(3dB) is in inverse proportion to the multiplication factor M, aproduct of the 3-dB cut-off frequency f_(3dB) and the multiplicationfactor M becomes constant. This product is called amultiplication-bandwidth product (MB product). The avalanche photodiodecan be operated at a higher speed as this product value becomes larger.

[0099] The 40 Gbit/sec trunk network needs an avalanche photodiode of abandwidth of 28 GHz that corresponds to 70% of a bit rate and the MBproduct of 200 GHz such that it operates at the optimum multiplicationfactor of 7 times. However, the avalanche photodiode having such largeMB product has not been provided to the market yet.

[0100] It is understood from Equation (1) that there are two approachesas the approach of increasing the MB product.

[0101] The first approach is the approach of increasing the ratio k ofionization rates. Since the ratio k of ionization rates is decidedaccording to the material, the material must be changed to change thevalue. In the first embodiment, the carrier multiplying layer 15 isformed of InP. But there is AlInAs as the material that has the largerratio k of ionization rates than InP. However, since AlInAs has the highdeliquescence property and absorbs a moisture to cause the deteriorationof its characteristic, such AlInAs causes reduction in the reliabilityof the photodetector.

[0102] The second approach is the approach of reducing the filmthickness of the carrier multiplying layer 15. If this approach isemployed, the voltage that is larger than that in the first embodimentmust be applied to the carrier multiplying layer 15 in order to producea large number of electron-hole pairs in the thin carrier multiplyinglayer 15.

[0103]FIG. 16 is a graph showing the electric field distribution in thedepth direction of the semiconductor photodetector shown in FIG. 3,wherein an ordinate denotes a depth from the avalanche photodiode 17 andan abscissa denotes an electric field distribution E at that depth.Then, a solid line indicates the electric field distribution along aII-II line in FIG. 3 in the first embodiment in which the carriermultiplying layer 15 is not thinned, and a dot-dash line indicates theelectric field distribution along a II-II line when the carriermultiplying layer 15 is thinned.

[0104] As apparent from FIG. 16, if it is intended to apply the largevoltage to the thin carrier multiplying layer 15 as described above, thelarge voltage is also applied to the underlying photo absorbing layer12.

[0105] However, since a band gap of the photo absorbing layer 12 isnarrow, even if applied voltage to the photo absorbing layer 12 issmaller than applied voltage to the carrier multiplying layer 15, theavalanche multiplication is caused in the photo absorbing layer 12. As aresult, the effective multiplication area width is spread up to thephoto absorbing layer 12 although the carrier multiplying layer 15 isthinned. Thus, the w in equation (1) is increased and rather the MBproduct is lowered.

[0106] Hence, even though the above two approaches are employed, it isimpossible to attain the high MB product although the highly-reliableInP is employed as the carrier multiplying layer 15.

[0107] In view of these respects, in order to attain the high MB productby the different approach from the above, the inventors of thisapplication carried out various experiments as follows.

[0108]FIG. 17 is a sectional view showing a planar avalanche photodiodeaccording to the present embodiment, which is manufactured for thisexperiment.

[0109] In this avalanche photodiode, an N-type semiconductor layer 32 isepitaxially grown on an N-type InP substrate 31 to have a thickness of 2μm. The N-type semiconductor layer 32 is made of InP into which Si isdoped as the N-type impurity at a concentration of 1×10¹⁸ cm⁻³. Then, aphoto absorbing layer 33 made of undoped InGaAs is epitaxially grown onthe N-type semiconductor layer 32 to have a thickness of 0.2 μm. Inorder to achieve the lattice matching between the photo absorbing layer33 and the underlying N-type semiconductor layer 32, a composition ratioof In and Ga is set to In:Ga=0.53:0.47 in InGaAs constituting the photoabsorbing layer 33.

[0110] In addition, an undoped InGaAs_(x)P_(1-x) layer is epitaxiallygrown as a heterobarrier relaxing layer 34 on the photo absorbing layer33. A composition wavelength of the undoped InGaAs_(x)P_(1-x) layer is1.25 μm, and a thickness thereof is about 0.1 μm, and an x value thereofis reduced gradually from 1.

[0111] Then, an N-type InP is epitaxially grown as an N-type fielddropping layer 35 on the heterobarrier relaxing layer 34 to have athickness of 0.02 μm. In the N-type InP, Si is doped at a concentrationof 1×10¹⁸ cm⁻³ as the N-type impurity.

[0112] A carrier multiplying layer 36 made of undoped InP and isepitaxially grown on the N-type field dropping layer 35 to have athickness of 0.05 μm. Then, a P-type semiconductor layer 37 isepitaxially grown thereon. The P-type semiconductor layer 37 is made ofP-type InP into which Zn is doped at a concentration of 1×10^(18 cm) ⁻³as the P-type impurity.

[0113] Then, a Ti/Pt/Au layer is formed on this P-type semiconductorlayer 37 by laminating an Au (gold) layer of 2 μm thickness, a Pt(platinum) layer of 0.2 μm thickness, and a Ti (titanium) layer of 0.03μm thickness in this order. Then, a P-side electrode 38 is formed onthis P-type semiconductor layer 37 by patterning the Ti/Pt/Au layer intoa ring-like planar shape.

[0114] Also, an AuGe/Au layer is formed as an N-side electrode 30 on onemajor surface of two major surfaces of the InP substrate 31 located onthe opposite side to the side where the P-side electrode 38 is formed.The AuGe/Au layer is formed by laminating an Au layer of 0.27 μmthickness and an AuGe layer of 0.03 μm thickness in this order.

[0115] Since functions of above respective layers 33 to 37 are equal tothose of respective layers 12 to 16 explained in the first embodimentrespectively, their explanation will be omitted herein.

[0116]FIG. 18 is a graph showing experimentally how the MB product ofthe avalanche photodiode is changed when a film thickness of the carriermultiplying layer 36 made of InP is varied. In this experiment, athickness of the photo absorbing layer 33 is also varied so that thethickness of the photo absorbing layer 33 is set to 0.2 μm in anexperimental value {circle over (1)} in FIG. 18 and the thickness of thephoto absorbing layer 33 is set to 0.8 μm in an experimental value{circle over (2)}. Also, for comparison, the MB product values that arepredicted based on Equation (1) are also depicted in FIG. 18.

[0117] As shown in FIG. 18, resultant experimental values become smallerthan a theoretical value calculated by Equation (1) in an area where thefilm thickness of the carrier multiplying layer 36 is thick. Incontrast, when the film thickness of the carrier multiplying layer 36becomes thinner than 0.1 μm, the MB product value of 200 GHz that isnecessary for the 40 Gbit/sec trunk communication can be obtained andthis value becomes larger than the theoretical value.

[0118] In this manner, if the film thickness of the carrier multiplyinglayer 36 is reduced, the large MB value can be obtained. However, if thefilm thickness of the carrier multiplying layer 36 is excessivelyreduced, the high voltage must be applied to the carrier multiplyinglayer 36 to cause the electron-hole pair production effectively. As aresult, it is possible that a tunnel current which flows through thecarrier multiplying layer 36 is increased.

[0119] The inventors of this application carried out the experiment tocheck how a value of the tunnel current flowing through the carriermultiplying layer 36 is changed correspondingly by varing the filmthickness of the carrier multiplying layer 36. The result is shown inFIG. 19.

[0120] As shown in FIG. 19, there is a tendency such that the tunnelcurrent is increased more largely as the film thickness of the filmthickness of the carrier multiplying layer 36 is reduced smaller.

[0121] Practically the tunnel current must be suppressed smaller than 1μA. It is understood from FIG. 19 that, in order to satisfy this value,a lower limit of the film thickness of the carrier multiplying layer 36should be set to 0.07 μm.

[0122] Meanwhile, FIG. 20 is a graph showing experimentally how the MBproduct is changed when the film thickness of the photo absorbing layer33 is varied. In this experiment, the film thickness of the carriermultiplying layer 36 is fixed to 0.1 μm.

[0123] As shown in FIG. 20, there is a tendency such that the MB productis increased as the film thickness of the photo absorbing layer 33 isreduced. Such increasing tendency becomes particularly conspicuous whenthe film thickness of the photo absorbing layer 33 is thinned smallerthan 0.2 μm. Accordingly, it is appreciated that, in order to attain thelarge MB product, the film thickness of the photo absorbing layer 33should be thinned smaller than 0.2 μm.

[0124] By the way, if the film thickness of the photo absorbing layer 33is excessively reduced to increase the MB product, it is possible that aquantum efficiency of the electron-hole pair production in the photoabsorbing layer 33 is lowered.

[0125] The inventors of this application carried out the experiment tocheck how the quantum efficiency in the photo absorbing layer 33 ischanged when the film thickness of the photo absorbing layer 33 isvaried. The result is shown in FIG. 21.

[0126] As shown in FIG. 21, the quantum efficiency tends to become loweras the film thickness of the photo absorbing layer 33 is reducedsmaller. Practically the quantum efficiency that is in excess of 0.7 isneeded. It is understood from FIG. 21 that, in order to satisfy thisvalue, the film thickness of the photo absorbing layer 33 should be setto 0.15 μm or more.

[0127] According to the above experimental results in FIG. 18 to FIG.21, it is apparent that, if the film thickness of the carriermultiplying layer 36 is set to more than 0.07 μm but less than 0.1 μmand the film thickness of the photo absorbing layer 33 is set to morethan 0.15 μm but less than 0.2 μm, the large MB product that is notpredicted from Equation (1) can be derived.

[0128] The inventors of this application considered the reason why suchadvantage can be achieved, as follows. First, in InP, the dead space ofthe hole is longer than the dead space of the electron and is almost0.01 μm. Therefore, it may be considered that, if the film thickness ofthe carrier multiplying layer 36 made of InP is reduced almost severaltimes the dead space of the hole, the scattering number of times of thehole in the carrier multiplying layer 36 becomes smaller than that ofthe electron, and thus it is considered that the ionization rate of thehole becomes larger than the ionization rate of the electron. As aresult, it is supposed that k in Equation (1) is increased and the MBproduct is increased.

[0129] Also, if one carrier is particularly observed, the carrier driftsmore quickly than its saturation velocity in an interval from thescattering of the carrier at a time point to the subsequent scattering.Therefore, it is supposed that, if the scattering number of times of thehole is reduced as described above, the saturation velocity v of thehole is increased. As a result, it is considered that the MB product isincreased according to Equation (1).

[0130] According to the avalanche photodiode having such large MBproduct, the photo current which follows up the light signal beingmodulated at a high speed such as about 40 Gbit/sec can be extracted,and thus it can contribute largely to the implementation of the trunknetwork in the next generation optical communication.

[0131] In the above, the explanation is performed with regard to theplanar avalanche photodiode shown in FIG. 17. But the present embodimentis not limited to this, and the present embodiment is also applied tothe avalanche photodiode 17 in the lateral-type incident structureexplained in the first embodiment in FIG. 3. In such case, if the filmthickness of the carrier multiplying layer 15 is set to more than 0.07μm but less than 0.1 μm and the film thickness of the photo absorbinglayer 12 is set to more than 0.15 μm but less than 0.2 μm, there can beobtained the semiconductor photodetector having the lateral-typeincident structure, in which the avalanche photodiode 17 is coupled tothe optical waveguide 5 and it can be operated at a high speed such asabout 40 Gbit/sec.

[0132] As described above, according to the semiconductor photodetectorof the present invention, the avalanche photodiode is formed in place ofthe PIN photodiode in the prior art. It leads to extracting the largerphoto current than the prior art and increasing the receivingsensitivity.

[0133] Also, in such semiconductor photodetector, the side surface ofthe underlying layer of the first conductivity type field dropping layercomes into contact with the side surface of the optical waveguide corelayer, and a part of the overlying layer of the first conductivity typefield dropping layer is formed on the optical waveguide core layer.Therefore, the breakdown voltage at the edge portion of the avalanchephotodiode can be enhanced, and thus the breakdown is difficult tooccur.

[0134] In addition, the composition of the underlying layer of the firstconductivity type field dropping layer is changed from its lower surfaceto its upper surface to function as the heterobarrier relaxing layer.Therefore, it can be prevented that the carrier is trapped by theunderlying layer to lower the response speed of the semiconductorphotodetector.

[0135] Further, according to the avalanche photodiode of the presentinvention, the film thickness of the photo absorbing layer is set tomore than 0.15 μm but less than 0.2 μm and the film thickness of thecarrier multiplying layer is set to more than 0.07 μm but less than 0.1μm. Therefore, the value of the MB product can be increased more largelythan the value that is predicted theoretically, and also the operatingspeed of the avalanche photodiode can be accelerated.

What is claimed is:
 1. A semiconductor photodetector comprising: (i)asemiconductor substrate; (ii)an optical waveguide having (a)a firstconductivity type semiconductor layer formed on the semiconductorsubstrate, (b)an optical waveguide core layer formed on a partial areaof the first conductivity type semiconductor layer, and (c)an uppercladding layer formed on the optical waveguide core layer; and (iii)anavalanche photodiode coupled to the optical waveguide, constructed byforming in sequence on another area of the first conductivity typesemiconductor layer (a)a photo absorbing layer, (b)a heterobarrierrelaxing layer, an underlying layer of a first conductivity type fielddropping layer, (c)an overlying layer of the first conductivity typefield dropping layer, (d)a carrier multiplying layer, and (e)a secondconductivity type semiconductor layer; wherein a side surface of theunderlying layer of the first conductivity type field dropping layercomes into contact with a side surface of the optical waveguide corelayer, and a part of the overlying layer of the first conductivity typefield dropping layer is formed on the optical waveguide core layer.
 2. Asemiconductor photodetector according to claim 1, wherein a filmthickness of the carrier multiplying layer is set to more than 0.07 μmbut less than 0.1 μm, and a film thickness of the photo absorbing layeris set to more than 0.15 μm but less than 0.2 μm.
 3. A semiconductorphotodetector according to claim 1, wherein composition of theunderlying layer of the first conductivity type field dropping layer ischanged from a lower surface to an upper surface to function as theheterobarrier relaxing layer.
 4. A semiconductor photodetector accordingto claim 1, wherein a band gap of the heterobarrier relaxing layer islarger than a band gap of the photo absorbing layer, and a band gap ofthe carrier multiplying layer is larger than a band gap of theheterobarrier relaxing layer.
 5. A semiconductor photodetector accordingto claim 1, wherein the underlying layer of the first conductivity typefield dropping layer is formed of N-type InGaAs, and the overlying layerof the first conductivity type field dropping layer is formed of N-typeInP.
 6. A semiconductor photodetector according to claim 1, wherein thesemiconductor substrate is an InP substrate, the first conductivity typesemiconductor layer is an N-type InP layer, the photo absorbing layer isan undoped InGaAs layer, the heterobarrier relaxing layer is an undopedInGaAsP layer, the carrier multiplying layer is an undoped InP layer,the optical waveguide core layer is an undoped InGaAsP layer, and theupper cladding layer is an InP layer.
 7. An avalanche photodiodecomprising: (i)a semiconductor substrate; (ii)a photo absorbing layerformed over the semiconductor substrate and having a film thickness ofmore than 0.15 μm but less than 0.2 μm; and (iii)a carrier multiplyinglayer formed over the photo absorbing layer and having a film thicknessof more than 0.07 μm but less than 0.1 μm.
 8. An avalanche photodiodeaccording to claim 7, further comprising: a heterobarrier relaxing layerformed on the photo absorbing layer; and a field dropping layer formedon the heterobarrier relaxing layer; wherein the carrier multiplyinglayer is formed on the field dropping layer.
 9. An avalanche photodiodeaccording to claim 7, wherein the photo absorbing layer is an undopedInGaAs layer, and the carrier multiplying layer is an undoped InP layer.